Novel Fluorescent Cyclic Nucleotide Derivatives to Study CNG and HCN Channel Function

A highly specific molecular interaction of diffusible ligands with their receptors belongs to the key processes in cellular signaling. Because an appropriate method to monitor the unitary binding events is still missing, most of our present knowledge is based on ensemble signals recorded from a big number of receptors, such as ion currents or fluorescence changes of suitably labeled receptors, and reasoning from these data to the ligand binding. To study the binding process itself, appropriately tagged ligands are required that fully activate the receptors and report the binding at the same time. Herein, we tailored a series of 18 novel fluorescent cyclic nucleotide derivatives by attaching 6 different dyes via different alkyl linkers to the 8-position of the purine ring of cGMP or cAMP. The biological activity was determined in inside-out macropatches containing either homotetrameric (CNGA2), heterotetrameric (CNGA2:CNGA4:CNGB1b), or hyperpolarization-activated cyclic nucleotide-modulated (HCN2) channels. All these novel fluorescent ligands are efficient to activate the channels, and the potency of most of them significantly exceeded that of the natural cyclic nucleotides cGMP or cAMP. Moreover, some of them showed an enhanced brightness when bound to the channels. The best of our derivatives bear great potential to systematically analyze the activation mechanism in CNG and HCN channels, at both the level of ensemble and single-molecule analyses.